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Nmap: Firewall configuration testing

This tip, in our Nmap manual series, explains how Nmap can be used to test the effectiveness of a firewall configuration. Learn how to use the open source network mapper to better understand how your firewall handles uninvited traffic and to test your firewall's ability to cope with fragmented traffic.

This is the sixth in a series of tips on how to use Nmap in an enterprise network environment.

So far in this series, we've looked at how Nmap can be used for network mapping and port scanning. In this tip we'll look at how Nmap can also be used to test the effectiveness of your firewall configuration

One of the best ways to understand how your firewall handles uninvited traffic is to verify that its filters and rules are working as you anticipated. For example, one mistake many administrators make when creating rules for allowing traffic through their firewall is to trust traffic based simply on its source port number, such as DNS replies from port 53 or FTP from port 20. To test whether your firewall allows all traffic through on a particular port you can use most of Nmap's TCP scans, including the SYN scan, with the spoof source port number option (--source-port or abbreviated just to –g). Simply provide a port number, and Nmap will send packets from that port where possible. For example, the following command will run a FIN scan using a spoofed source port number of 25 (SMTP) saving the output to file firewallreport.txt.

nmap -sF -g 25 -oN firewallreport.txt 

It's also worthwhile to test your firewall's ability to cope with fragmented traffic. Attackers often split up the TCP header over several packets to make it harder for packet filters and intrusion detection systems to detect an attack. While fragmented packets won't get past packet filters and firewalls that queue all IP fragments, many devices have queuing disabled by default to avoid a drop in performance. Just add the -f option to set a scan to send fragmented IP packets.

When auditing your firewall, I recommend that you scan ports in numerical order (option -r) to make it easier to follow the traffic flow when analyzing the Nmap output files. However, when testing the effectiveness of firewalls and intrusion detection systems, use the default, which is a randomized port order, in addition to randomizing the order in which hosts are scanned by using the randomize-hosts option, which can be abbreviated to –rH. This, combined with slow timing options, which we will look at next week, will make any network monitoring devices you have work hard to detect the scan. An example command to test your network defenses could be: 

nmap -sS --scan-delay 500 -f -rH firewallreport.txt

Once you have identified any unfiltered ports or other issues, you should review your firewall rules to ensure that access to all services is controlled, and that filters and rules are performing as intended. After changes are implemented, run Nmap again to check that your changes achieve the desired effect. Nmap offers many features for circumventing poorly implemented firewalls, so play the role of an attacker and find out whether your network defenses can cope with FTP bounce scans, idle scans or fragmentation attacks.

Nmap technical manual

About the author:
Michael Cobb, CISSP-ISSAP, is a renowned security author with over 20 years of experience in the IT industry. He has a passion for making IT security best practices easier to understand and achievable. His website offers free security posters to raise employee awareness of the importance of safeguarding company and client data and of following good practices. He co-authored the book IIS Security and has written many technical articles for leading IT publications. Mike has also been a Microsoft Certified Database Manager and registered consultant with the CESG Listed Advisor Scheme (CLAS).

Next Steps

Get some tips and tricks to using Nmap to scan a network

Discover how Nmap 4.01 improves upon past releases

View a demonstration of using Nmap to scan a network

This was last published in July 2006

Dig Deeper on Network device security: Appliances, firewalls and switches